D-Aspartic acid is one of the two forms of the amino acid called Aspartic acid (a non-essential amino acid and one of the two components of aspartame, an artificial sweetener). The naturally occurring form of Aspartic acid is L-Aspartic acid. It is mainly sold as a testosterone booster, although there is inconclusive research to support that it actually can increase testosterone. Possible side effects may include an increase in estrogen (if it actually does increase testosterone).